Girl Drama

I am the one in the middle!  And sometimes it is not a great place to be!  Chelsea is 17 and I am the first to admit that my daughter sometimes has a very crappy attitude.  She can be mean, self centered, entitled and just an all out bitch.  It makes me crazy but I've learned to fight my battles and sometimes to just ignore her and let some things she says or does slide.  She is a moody hormonal teenage girl.  Now if my mother could do the same thing we would have no problem.  But can she?  Noooooooo . . . she feels the need to make comments that Chelsea hears and then Chelsea gets mad or get her feelings hurt.  Last night my mother came over for dinner and Chelsea was sitting at the counter doing her homework.  Chels did have an attitude and was yelling at the boys and the dogs - she was obnoxious.  My mother says that she is leaving and I asked her why.  She looked at Chelsea and whispered to me that Chelsea was being bitchy.  Well Chelsea heard her and was not happy.  I told my mother that she didn't need to make any comments.  When I came back into the room Chelsea was crying and said that she feels like my mother is always saying something about her.  And to be quite honest, my mother doesn't hold back much on how she is feeling.  Chelsea also feels that I didn't stick up for her.  I did say something to my mother last night but it is hard to say too much - she is MY mother.  And I don't want to hurt her feelings.  But I don't want her hurting my daughter's feelings either.  So now what?